Our previous work of hematopoitic stem/progenitor cells has shown that committed erythroid lineage is capable of switching into myeloid lineage cells, and vice versa, under a unique culture system with EPO or G-CSF stimulation. The phenotype of specific cells might be reflected by patterns of genes and proteins expressed in various stages of cellular development. To understand the possible mechanisms of these two lineage?s differentiation and switches, we have currently extended this study into the molecular events during the process of erythroid and myeloid development with five cell populations of E14, G14, E14?G14 and G14?E14 cells within the culture system. Cell population studies are more significant since they more accurately reflect the physiologic reality of continuously interacting cells. Using RAGE method, a total of 266 expressed gene-specific fragments were investigated, of which 65% (171) of total gene products were shared within D0, E14, and G14, 3%~9% of genes were observed characteristic for individual populations at 14-day point, and grouped into 11 different types of expression patterns. The co-expressed genes implicate a close development relationship between erythroid and myeloid lineages and a basis for these two lineages? conversion each other. 145 (55%) cDNA products have been identified with known genes of cell cycle regulators, transcription factors, and lineage specific genes. 27 (18%) of 145 fragments previously identified genes with unknown function, these genes may co-function with the known genes that appear in same biological process, and 3 (2%) were defined as novel sequences. The expression profile of protein spots using 2DE technique shows similar patterns to the gene expression distribution of cell populations. We have compared qualitative and quantitative expression profiles of intracellular proteins among the five cell populations. The data suggest there are some cell populations of being responsible specifically for cytokine?s signaling and exhibit a molecular phenomena that some cells are responsible for both EPO and G-CSF at same time, but exact contradict, rather than synergic, biological effect, which indicates a true gene reprogram happened and an important guidance for both basic study and therapeutic purpose with these two factors. Currently, gene products and protein spots, which were consistently up-regulated or down-regulated in primary and secondary culture cells with EPO or G-CSF, as well as the three novel genes have been chosen for further study and will be reported in future articles. This is the first time that the lineage development of erythroid and myeloid cells has been studied simultaneously at both mRNA and protein levels. The work indicates that there is a much more close development relationship between erythroid and myeloid lineages than thought before, and gene reprogram (transdifferentiation) may take place during these two lineage cellular differentiation and switches. Understanding mechanisms of hematopoietic differentiation and switch will lead to valuable insights.